To be able to efficiently operate electrical machines which have highly variable and cyclic power consumption, it is necessary to provide energy stores which can store high amounts of energy and can receive and output them at short notice. Such demands are made, for example, with self-propelled work machines which comprise electrical traction drives, for example mobile construction machinery, but also stationary plant which at times cannot be connected to the public grid due to highly changing power consumption and which require their own generator. Such energy stores are, however, generally also sensible for on-grid machines having electrical drives if they have a highly variable and cyclic power consumption.
Electrical energy storage elements such as capacitors which allow fast charging and discharging, however, typically have a limited store of energy which is not sufficient for applications with high amounts of energy to be stored. It is therefore usually necessary to connect such energy storage elements to form storage modules. To be able to achieve high voltage values in this respect, a plurality of energy storage elements have to be connected in series with low nominal voltage values of the individual elements.
It must, however, in particular be ensured when charging electrical energy storage elements connected in series that not a single one of the energy storage elements is overloaded voltage-wise. There can be a risk of an overload of an individual element both with storage batteries and with capacitors if said individual element has already been charged to the nominal voltage when charging, but other elements are still considerably remote from their nominal voltage and therefore have to be charged further.
Double-layer capacitors which do not have an electrically insulating dielectric, but which typically do have an electrolyte between their two electrodes are particularly powerful per se, but are also provided with a very small nominal voltage. The storage of the electrical energy takes place here by charge transfer at the boundary surfaces, whereby large capacity values are achieved, but the permitted voltage is simultaneously limited to a few volts. If an energy store is built up of a plurality of such capacitors which are connected in series, no single capacitor may exceed its permitted voltage maximum since otherwise irreversible changes to the electrolyte would take place.
Baluns can be used to utilize the permitted energy content of the storage elements as much as possible, but, on the other hand, to avoid any overcharging or overvoltages. If the storage cell voltages of a storage block are balanced, the storage cells age in the same manner and the block is given its longest service life in this manner. With fully charged double-layer capacitor storage cells, an additional voltage of only 0.1 V can already halve the service life.
There are different approaches for such baluns. A distinction must first be made between passive solutions and active solutions. Whereas in passive solutions, energy is converted into heat and the cell voltage is thus lowered at too high a cell voltage, active solutions can produce a charge balance between the cells with a low voltage and those with a high voltage.
Such active solutions are available with capacitive or with inductive circuit components. In capacitive processes, a capacitor is typically connected across the cell having the highest voltage, with this capacitor then being placed across the cell having the lowest voltage. In the inductive solutions, a balance is established across a number of individual transformers or across a common transformer having multiple windings.
Different already known baluns for storage cells connected in series are known, for example, from the documents DE 100 39 407 A1, DE 10 2005 034 588 A1, DE 10 2009 041 005 A1 or DE 10 2008 048 382 A1.
With the known inductive, active baluns which produce a charge balance between the cells having a low voltage and the cells having a high voltage with inductive components, the previous solutions are capable of improvement in various aspects. If only one common balancer transformer is used for the storage cells connected in series due to the high price of a large number of single transformers, a further common coil which is connected to the output voltage of the storage block is typically provided beside the plurality of individual windings, one each per storage cell.
In this respect, energy can be taken from the sum voltage of the storage block and a switching power supply can be built, for example in the form of a flyback, which feeds the stored energy into each individual cell via diodes. The diodes can achieve that the cell having the lowest voltage receives most of this energy so that a balance is brought about. On the other hand, a switching power supply can also be built with the energy of the storage cells, with the inductive discharge being centrally fed into a sum rail. In this case, the storage cell having the highest voltage contributes most to the current flow so that its voltage is lowered. In this respect, the transformer likewise has to have a further coil beside the individual coils which has a winding number which is calculated from the number of windings of the individual coil times the number of storage cells.
In both cases, the common balancer transformer requires, beside the n coils for the n storage cells, an additional coil having a high number of windings which is to be calculated for a respective specific number of cells and which changes when a different number of cells is used.
In the named processes which transport energy from the storage cells via a common transformer into other cells or into a sum rail, the balance current flow is typically not limited. However, this has the result that the current can adopt a non-defined high value which depends on the voltage difference of the individual cells. Accordingly, the electronic switch for the individual cell, in particular the corresponding transistor, has to be of very large dimensions since its maximum current is not fixed. The cables from the balun to the taps of the storage block furthermore also require a higher cross-section. The plug systems become more expensive.
Starting from this, it is the underlying object of the present invention to provide an improved electrical energy storage apparatus of the initially named type which avoids disadvantages of the prior art and further develops the latter in an advantageous manner. An efficient, simply built inductive balun should in particular be provided whose balancer transformer has a simple and inexpensive design and whose switches can be controlled simply and are only exposed to small amounts of energy.
The named object is achieved in accordance with the invention by an electrical energy storage apparatus having a plurality of electrical storage cells connected in series and having an inductive balun for balancing voltages of the storage cells, wherein the balun has a balancer transformer having separate coils for the storage cells and has a respective electric switch for each storage cell, wherein the coils of the balancer transformer are connected via at least one respective inductance and via the electric switch connected to the named inductance to a pole of the respective storage cell, with the electric switch associated with a respective storage cell being connected via a diode to a pole of a respective storage cell arranged upstream or downstream.
Provision is therefore made in accordance with the invention that the coils of the balancer transformer are connected to a pole of the respective storage cell via a respective at least one inductance and a switch connected to the named inductance, with the switch associated with the one respective storage cell being connected via a diode to a pole of a storage cell respectively arranged upstream or downstream. Balance currents can be limited via the named diodes and the switches can be operated with limited currents or voltages.
In this respect, in a further development of the invention, the circuit components associated with a respective storage cell and comprising an electrical switch, the potential-separated coil of the balancer transformer, an inductance and a diode can in particular be connected to one another such that a first terminal of the named coil is respectively connected to the positive pole of the associated storage cell, a second terminal of the named coil is connected to a first terminal of the inductance, the second terminal of the inductance is connected to a first terminal of the aforesaid switch, the second terminal of the switch is connected to the negative pole of the associated storage cell, the anode of the aforesaid diode is connected to the first terminal of the switch and the cathode of the diode is connected to the positive pole of that storage cell which follows as the next in the positive direction in the serial connection. The diode connected to the switch of a first storage cell can therefore be connected to the positive pole of the second storage cell which follows the first storage cell as the next in the positive direction. It could generally also be considered to jump over one or more storage cells in the connecting of the diode, that is to connect the diode connected to the switch of the first storage cell to the positive pole of the third or fourth storage cell. The aforesaid connection to the directly next storage cell in the positive direction, however, allows a more simple wiring and a more uniform voltage balancing.
In accordance with another further development of the invention, the order in the serial connection of the transformer winding and the inductance can be reversed, in particular such that the first terminal of the inductance is respectively connected to the positive pole of the associated storage cell and the second terminal of the transformer winding is connected to the first terminal of the switch.
In the topmost storage cell, which is connected to the positive pole of the storage block, and indeed in particular directly without the interposition of further storage cells, the inductance can be formed as a storage choke, with the named storage choke being able to comprise two coils which are the same. In this respect, a first coil of this storage chock can advantageously be connected in accordance with the inductances in the other storage cells, whereas the second coil of the named storage choke advantageously leads in series with a further diode to the respective positive terminal or negative terminal of the bottommost storage cell which contacts the negative terminal of the storage block. In this respect, the cathode of this further diode can advantageously face toward the positive pole of the lower storage cell.
The balun advantageously manages with only one balancer transformer which is associated with all storage cells together, with the named transformer being able only to have coils with the same number of windings, with the coil advantageously being able to be designed using a commercial ribbon cable. For the arrangement of n—for example 24—storage cells, n—for example then 24—separate coils are provided, which can be implemented easily and inexpensively by a corresponding n-pole—for example 24-pole—ribbon cable.
In a further development of the invention, the balancer transformer can be formed from a transformer core around which windings of a ribbon line are wound. Each lead of the ribbon line can advantageously form a galvanically separated or a potential-separated coil on the balancer transformer.
Such a ribbon line of the balancer transformer having conventional flat ribbon plugs can advantageously be guided and connected or contacted on a printed circuit board at which further components of the balun are provided.
The named balancer transformer can advantageously have a plurality of such coils with ribbon lines.
The core of the balancer transformer can, in a further development of the invention, at least partly comprise a highly permeable ferrite material. In a further development of the invention, the core of the balancer transformer can at least partly comprise nano-crystalline and/or amorphous ferrite materials.
Provision can be made in an advantageous embodiment of the invention that with respect to the switches associated with the storage cells, only one switch per storage cell is provided.
The electric switches can generally be configured in different manner in this respect. In a further development of the invention, at least one of the electric switches is designed as a transistor with all the electric switches being able to be designed as transistors in a further development of the invention.
In a further development of the invention, at least one electric switch, in particular also every electric switch, can be formed as a MOSFET or as an IGBT transistor.
A further advantageous embodiment of the invention can comprise that at least one electric switch, in particular every electric switch, is designed as a bipolar NPN transistor having an anti-parallel diode.
In a further development of the invention, a uniform control, which can in particular be implemented in the form of only one single control circuit, can be provided for a plurality of electric switches, preferably all the electric switches, which are associated with the storage cells.
In an advantageous further development of the invention, the control terminals of a plurality of electric switches, in particular of all the electric switches, can be acted on by a constant signal, in particular such that all the switches switch on simultaneously and/or switch off simultaneously in a synchronous manner.
Only in pulse generator circuit can advantageously be used to control all the control terminals.
The control of the electric switches can be separated from the storage block potential-wise or galvanically via a gate transformer and/or via optocouplers.
Comparators, operation amplifiers or other control circuit components can be dispensed with on the side of the storage elements.
The invention will be illustrated in more detail in the following with respect to a preferred embodiment and to an associated drawing.